Marco Rubio unleashed an attack on Ted Cruz. Donald J. Trump bought his first television ad, almost certainly to blunt the surge of Mr. Cruz. Chris Christie took off on Mr. Trump, suggesting the billionaire businessman wasn’t a “grownup.” Then Mr. Trump opened new questions about whether Mr. Cruz, born in Alberta, satisfied constitutional strictures for the presidency.

It’s a new year and suddenly it’s a new Republican campaign.

The posturing is over. The ranking of the candidates is in flux. The money is flowing into TV and radio outlets in New Hampshire and Iowa. And the campaign is gaining new clarity.

This was bound to happen as the Iowa caucuses, now only three weeks away, grew near. Even so, the transformation of the campaign from a parade of events into a riot of conflict took place with astonishing swiftness. It was as if the packing away of the holiday tinsel in the nation’s attic opened a trap door in the political edifice.

In Iowa, the new calculus adds new urgency to organizational work that, in some cases, has been underway for six months; the caucuses in that state are more a matter of mass mobilization than an exercise in mass media. In New Hampshire, which prides itself on its independence and seldom takes its cue from its prairie cousin, the new season means a frantic struggle for advantage among establishment-oriented candidates.

It is a matter of conviction — backed by survey data — that Granite State voters make their decisions late. Nearly half the Republicans who voted in the last primary decided in the final three days, which means five days after Iowa went to former Sen. Rick Santorum of Pennsylvania. Then Mr. Santorum finished fourth in New Hampshire, with less than a tenth of the vote, trailing even former Gov. John Huntsman Jr. of Utah.

The two early states present vastly different political profiles. While both are increasingly suburban, their characters were molded by geography, Iowa by the open plains and the farms that filled them after the great westward migrations of the 19th century, New Hampshire by the mountain fastnesses that, until the interstate highways of the late 1960s and the cable television infusion in the 1980s, gave the state a remote and rugged outlook and a distinctive political culture of somber conservatism and cheerless thrift.

Those different profiles may explain the differences in the campaigns in the two states.

Iowa is becoming a struggle between Mr. Trump and Sen. Cruz, who profited by appealing to the Trump base while others shunned or demeaned it even as he cultivated the religious conservatives who had fueled the triumphs of Mr. Santorum (2012) and former Gov. Mike Huckabee of Arkansas (2008) and, as far back as 1988, provided the Rev. Pat Robertson a second-place finish that dropped Vice President George H.W. Bush to a mortifying third.

But the Republican race in New Hampshire bears almost no resemblance to Iowa.

In the Granite State, according to the latest poll, Mr. Cruz is in fifth place, eclipsed (though within the margin of error) by two governors, John Kasich of Ohio and Chris Christie of New Jersey.

Mr. Trump still holds the whip hand — all New England is still talking about the crowd he attracted on a cold night in nearby Lowell, Mass., last week — but oftentimes the significance of New Hampshire is not about who wins but who exceeds expectations or emerges.

It was not President Lyndon B. Johnson’s 49 percent in 1968 that drew the nation’s attention but the 42 percent polled by anti-war candidate Sen. Eugene McCarthy of Minnesota. It was not the 46 percent won by Sen. Edmund S. Muskie of Maine in 1972 that mattered but the 37 percent polled by Sen. George S. McGovern of South Dakota. And it wasn’t the 33 percent won by former Sen. Paul E. Tsongas of Massachusetts that mattered in 1992 but the 25 percent polled by Gov. Bill Clinton of Arkansas.

In each case, the winner didn’t become the Democratic nominee. Similarly, this year the attention isn’t so much on who wins — the Real Clear Politics running average of New Hampshire polls has Mr. Trump comfortably in first place, with double the support of his nearest rival, Mr. Rubio — but on who comes in second and third.

That’s because the race for second and third in New Hampshire is the race for the mainstream alternative to Mr. Trump and Mr. Cruz. And that’s why Mr. Christie and Mr. Kasich, accustomed to running statewide campaigns, are essentially running gubernatorial campaigns, patterning their efforts to become chief executive of the country on their earlier successes in running for chief executive of their states.

All this underlines the unusual architecture of American presidential politics, which propels into national office political figures who win small slivers of votes in states that bear almost no resemblance to the nation as a whole. New Hampshire, for example, is 1.1 percent African-American, while African-Americans account for 13.2 percent of the nation. Iowa is 51 percent rural, while the nation as a whole is 19 percent rural.

And that is without taking into consideration the political differences between those who participate in primaries and caucuses as compared to the nation as a whole. Just as the Democratic primary voter generally leans leftward, the Republican primary voter is generally more conservative than the GOP general-election voter.

“This system means that small groups of hardliners of both parties decide who gets on the ballot,” says Mickey Edwards, a Republican who served in the U.S. House from Oklahoma for 16 years, was national chairman of the American Conservative Union and who has taught at Harvard and Princeton universities. “We’re always choosing among candidates who don’t represent the country as a whole.”

Mr. Edwards warns that the nation could end up with at least one presidential nominee who never won a majority of the primary or caucus vote in any state. “Our system,” he argues, ‘’is seriously, seriously flawed.”

Perhaps he is right. But in the next four weeks — New Hampshire follows Iowa by a mere eight days — the candidates who exploit those flaws with the greatest skill may move closer to the White House, regardless of how flawed they are themselves.

David Shribman, a Pulitzer Prize winner in journalism, is executive editor of the Pittsburgh Post-Gazette.